Controls B67 Gas Valve. Need to replace 2 wire thermostat. Hate to sound stupid but can not see anywhere on this furnace or on the newer (but still old) gas valve where you turn off the power? Do I just turn it to off and then can replace the upstairs thermostat?

You might have the old style millivolt generator pilot. The pilot actually generates the small amount of electric power the system needs to operate and there for it will not have a regular electric line from your electric panel attached. If you turn the pilot off that will stop the flow of electric.

NOTE:If you are going to replace the thermostat you will need to get one that will work with a millivolt system. It should be marked on the box on on the instructions if it is compatible. Many thermostats for sale now days will not work with your system.

Another note: The furnace you have is a real gas hog. Have you ever considered replacing it with a newer high efficiency unit? The reason I ask is because a new high efficiency furnace will more than cut your gas bill in half and that is big buck savings for most people.

Thank you for the quick reply and confirmation I won't be zapping myself. I did purchase a lux $16 thermostat that handles 24 Millivolt systems. The only reason I have not replaced the furnace is that I only use it maybe 20 days a year living in California. I have a gas fireplace that suffices most times. I would love to get rid of all that huge duct work up to the rooms of my 80 year old house,

WOOPS ((thermostat that handles 24 Millivolt systems)) Does the thermostat say millivolt or 24 volt. If 24 volt and not millivolt it will not work. @4 volt thermostats use a transformer to generate the voltage a millivolt system does not. Just checking.

Woops it is a 24 Volt/Millivolt (from package) and its working like a charm. Thank you very much for your help!

OK Then the thermostat was good for both 24 volt and millivolt systems. Good deal glad you go it going.